The roles of morphogenetic differentiation in controlling the phenotypic expression of neoplastic transformation, the degree of malignancy of tumors, and the susceptibility of developing organs to carcinogenesis are studied using organ culture and tissue transplantation techniques, with current emphasis on the kidney. A serum-free culture system has not been devised that allows for both growth and branching of ureteric bud and proliferation of metanephric mesenchyme. Tubulogenesis has been observed in mesenchyme maintained under these conditions in the absence of an inducing tissue. The ability of transplacentally administered car- cinogens to induce genotoxic damage in cells of embryos or fetuses exposed at different stages of gestation was determined for rat, mouse, and Syrian hamster. Cells were isolated from exposed embryos and gene mutation at three loci (resistance to 6- thioguanine (6-TG) and ouabain and to diphtheria toxin (DT) in the hamster) were assayed in vitro with simultaneous determination of survival ability. Induced DT and TG mutant frequency (mutants per surviving cell) was greatest when treatment was day 9 of gestation for mesenchymal cells of the Syrian hamster fetus. Cells from the brain also had a large increase in frequency of mutants when fetus were exposed to NEU at day 9 of gestation and also appeared to have another peak of sensitivity at day 6-7 of gestation. In addition, an almost completed study has as its objective to determine the number of mutants induced in individual fetuses which have been treated with NEU at different days of gestation. Another study aims determined the spontaneous mutation frequencies Syrian hamster using litters of day 13 fetuses. This was 2 X 10-7 DT mutants per viable cell and 2 X 10-7 TG resistant mutants per viable cell. 95 and 99% confidence limits were calculated for these mean spontaneous frequencies in order to compare with cells from litters which have been transplacentally treated with a mutagen. In addition the number of 6-TGr mutant cells was determined in a large number of individual day 13 fetuses. Mutants could not be detected in half of these fetuses, despite testing an average of 53 plates per fetus.